Polymer Chemistry

Stress-Strain Curves⁴⁴

The tensile stress on a material is defined as the force per unit area as the material is stretched. The cross-sectional area may change if the material deforms as it is stretched, so the area used in the calculation is the original undeformed cross-sectional area A_o.

stress:

The units of stress are the same as those of pressure.We will use pascals, Pa, as the units for the stress. In the polymer literature, stress often is expressed in terms of psi (pounds per square inch).

The strain is a measure of the change in length of the sample. The strain commonly is expressed in one of two ways.

elongation:
extension ratio:

The strain is a unitless number.

A tensile stress-strain curve is a plot of stress on the y-axis vs. strain on the x-axis. In the plot at the right, strain is expressed as elongation. Stress-strain curves are measured with an instrument designed for tensile testing.

We see that as the strain (length) of the material increases, a larger amount of stress (force) is required.

As the elongation is increased the sample eventually breaks.